feat(ui): 添加底部状态栏和优化代码编辑器空状态

- 新增 StatusBar 组件，显示 Git 分支、诊断信息和连接状态
- 添加 Git API 端点 (GET /api/files/git) 获取分支和 dirty 状态
- 优化 CodeEditor 空状态，添加图标和引导提示
- 修复 Chat 页面高度问题 (h-screen -> h-full)

packages/ui/src/components/CodeEditor.tsx

@@ -13,7 +13,7 @@ import { html } from '@codemirror/lang-html';
 import { css } from '@codemirror/lang-css';
 import { markdown } from '@codemirror/lang-markdown';
 import { oneDark } from '@codemirror/theme-one-dark';
-import { X, Save, Circle } from 'lucide-react';
+import { X, Save, Circle, FileCode, MousePointerClick } from 'lucide-react';
 import { motion, AnimatePresence } from 'framer-motion';
 import { toast } from 'sonner';
 import { cn } from '../utils/cn.js';
@@ -156,8 +156,25 @@ export function CodeEditor({
 
   if (tabs.length === 0) {
     return (
-      <div className={cn('flex items-center justify-center h-full bg-surface-base text-fg-muted', className)}>
-        <p>No files open</p>
+      <div className={cn('flex flex-col items-center justify-center h-full bg-surface-base', className)}>
+        <motion.div
+          initial={{ opacity: 0, y: 10 }}
+          animate={{ opacity: 1, y: 0 }}
+          transition={{ duration: 0.3 }}
+          className="text-center max-w-xs"
+        >
+          <div className="w-16 h-16 mx-auto mb-4 rounded-2xl bg-surface-subtle flex items-center justify-center">
+            <FileCode size={32} className="text-fg-subtle" />
+          </div>
+          <h3 className="text-fg font-medium mb-2">No files open</h3>
+          <p className="text-fg-muted text-sm mb-4">
+            Select a file from the explorer to view and edit its contents
+          </p>
+          <div className="flex items-center justify-center gap-2 text-xs text-fg-subtle">
+            <MousePointerClick size={14} />
+            <span>Click a file to open</span>
+          </div>
+        </motion.div>
       </div>
     );
   }